Non-uniform overlapping printing

ABSTRACT

A printing method drives each of the n(j) number of nozzles to enable the nozzle to form dots intermittently at a rate of one in m×q dot positions on the j-th main scan line during one main scan, where m is an integer of 1 or more, and q is an integer of 2 or more, to thereby complete dot formation on the j-th main scan line with the n(j) number of nozzles during n(j) number of main scans. The printing method attains high recording speed and high image quality, comparing to cases when all main scan lines are recorded using a fixed number of nozzles.

This application is a Continuation-in-part (CIP) of application Ser. No.10/137,423 filed on May 3, 2002, which is a Continuation of applicationSer. No. 09/840,132, filed Apr. 24, 2001, now U.S. Pat. No. 6,390,598.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a technology that performs printing byforming dots on a printing medium using a printing head.

2. Description of the Related Art

Inkjet printers such as serial scan type printers and drum scan typeprinters perform printing using a printing head while scanning in a mainscan direction, and form text and images on a printing medium byejecting ink from multiple nozzles of a printing head.

As one dot recording method used with inkjet printers, there is a methodcalled the “interlace method.” FIG. 31(A) is an explanatory diagram thatshows sub-scan feed for an interlace recording method. Printing head 10has four nozzles placed along the sub-scanning direction. The numbers 0through 3 noted in the circles are the nozzle numbers. Nozzle pitch k inthe sub-scanning direction between nozzles is 3 dots. Here, a unitcalled a “dot” means a dot pitch in the sub-scanning direction thatcorrelates to a printing resolution in the sub-scanning direction. InFIG. 31(A), the positions of printing head 10 noted as pass 1, pass 2,etc. indicate the sub-scanning direction position at the time of eachmain scan. Here, “pass” means one main scan. After each main scan,sub-scan feed is executed at a four dot fixed feed amount L.

FIG. 31(B) shows the ordinal numbers of the nozzles that record dots oneach main scan line. As can be understand from this Figure, with theinterlace recording method, even when the nozzle pitch k is 2 dots orgreater, dots can be formed on all main scan lines.

However, the positions of dots formed by each nozzle sometimes shift alittle bit in the sub-scanning direction due to nozzle manufacturingerror. FIG. 31(B) shows a case where there is no such manufacturingerror and all the dot positions are normal. Meanwhile, in a case when adot formed by nozzle #1 is shifted vertically, for example, as shown inFIG. 31(C), a gap occurs between the main scan line formed by dots fromnozzle #1 and the main scan line formed by dots made by nozzle #0. Withthe naked eye, this kind of gap is observed as a stripe shaped area ofimage quality degradation called “banding.” Note that the cause ofbanding is not just nozzle manufacturing errors, but may include othererrors such as sub-scan feed error, folds in the printing medium, etc.

To prevent this kind of banding, a recording method called anoverlapping recording method is used. FIGS. 32(A) through 32(C) show theeffect of the overlapping recording method. As shown in FIG. 32(A), thesub-scan feed amount L for this recording method is a fixed value of twodots. In FIGS. 32(A) through (C), the nozzle positions of the evennumbered passes are shown by a diamond shape. When all the dot positionsare normal, as shown in FIG. 32(B), the dot position recorded on evennumbered passes are placed alternately with the dot positions recordedon odd numbered passes without any gaps. As a result, the dots on thesame main scan line are formed by two different nozzles. In this way,this method of recording using multiple different nozzles to recordmultiple dots on the same main scan line is called an “overlappingmethod.” The overlapping method also includes a method of recordingusing the same nozzle during multiple main scans to record multiple dotson the same main scan line in a broad sense.

With an overlapping method, as shown in FIG. 32(C), where dots formed bynozzle #1 are shifted vertically as well, we can see that the gap doesnot stand out as much as in FIG. 31(C). As a result, it is possible tosoften the banding. To make the most of this advantage of this kind ofoverlapping method, it is desirable to make the number of overlaps (inother words, the number of nozzles in charge of forming dots on eachmain scan line) as big a value as possible.

However, the overlapping method has the problem that printing speed isslower than with non-overlapping methods. The sub-scan feed amount L inthe non-overlapping method shown in FIG. 31 is four dots, while that inthe overlapping method shown in FIG. 32(A) is two dots. The printingspeed is approximately proportional to the sub-scan feed amount, so theprinting speed of this overlapping method is approximately half that ofthe non-overlapping method.

With inkjet printers, there is demand for high speed printing of imagesat the same level of smoothness as a photograph. Achieving higher imagequality can be achieved to some degree by making the dots smaller.However, when the dots are smaller, there is a tendency for the bandingdue to variance in nozzle characteristics to stand out more. Meanwhile,to soften this kind of banding, when the number of overlaps isincreased, there is the problem that printing speed is decreased.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to improve imagequality without excessively decreasing printing speed.

In order to attain the above and the other objects of the presentinvention, there is provided a method of printing by forming ink dots ona print medium. The method comprises the steps of: providing a printhead having a plurality of nozzles arrayed along a sub-scanningdirection for ejecting same ink; allocating n(j) number of nozzles to aj-th main scan line in a selected area on the print medium where n(j) isan integer of at least 2; positioning one of the n(j) number of nozzleson the j-th main scan line during each of k(j) number of main scans tobe performed on the j-th main scan line where k(j) is an integer of atleast 2, the integer k(j) for some main scan lines being set at adifferent value from that for other main scan lines; and driving the oneof the n(j) number of nozzles for each of the k(j) number of main scans,in response to given print data, to enable the nozzle to form dotsintermittently at a rate of one in mxq dot positions on the j-th mainscan line during one main scan, m being an integer of at least 1, and qbeing an integer of at least 2, to thereby complete dot formation on thej-th main scan line during k(j) number of main scans.

In the printing method of the present invention, the number of mainscans during which each of some main scan line is recorded differs fromthe number of main scans for each of the other main scan lines. Forexample, some main scan lines are recorded during four main scans whileother main scan lines are recorded during two main scans. Therefore, itis possible to increase the recording speed, comparing to cases when allmain scan lines are recorded during four main scans, and the imagequality can be improved comparing to cases when all main scan lines arerecorded during two main scans.

The present invention can be realized in various forms such as a methodand apparatus for printing, a method and apparatus for producing printdata for a printing unit, and a computer program product implementingthe above scheme.

These and other objects, features, aspects, and advantages of thepresent invention will become more apparent from the following detaileddescription of the preferred embodiments with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram that shows the structure of a printing systemas a embodiment of the present invention;

FIG. 2 is an explanatory diagram that shows the structure of theprinter;

FIG. 3 is a block diagram that shows the structure of control circuit 40in color printer 20;

FIG. 4 is an explanatory diagram of the nozzle array on the bottomsurface of printing head 28;

FIG. 5(A) shows an example of sub-scan feed on the basic conditions of anormal interlace recording method;

FIG. 5(B) shows the parameters of that dot recording on the basicconditions of a normal interlace recording method;

FIG. 6(A) shows an example of sub-scan feed on the basic conditions ofan overlapping interlace recording method;

FIG. 6(B) shows the parameters of that dot recording on the basicconditions of an overlapping interlace recording method;

FIG. 7 is a block diagram that shows the major structure of head drivecircuit 52;

FIG. 8(A) is a timing chart that shows the operation of head drivecircuit 52 for a non-overlapping interlace method;

FIG. 8(B) is a timing chart when dots are formed at odd numbered pixelpositions using an intermittent overlapping method;

FIG. 8(C) is a timing chart when dots are formed at even numbered pixelpositions using an intermittent overlapping method;

FIG. 9 is an explanatory diagram that shows the dot recording method ofthe first comparative example;

FIG. 10 is an explanatory diagram that shows the dot recording method ofthe first embodiment of the present invention;

FIG. 11(A) shows the allocation of raster data to each nozzle for thefirst comparative example of FIG. 9;

FIG. 11(B) shows the allocation of raster data to each nozzle for thefirst embodiment of FIG. 10;

FIG. 12 is an explanatory diagram that shows the dot recording method ofthe second embodiment of the present invention;

FIG. 13 is an explanatory diagram that shows the dot recording method ofthe third embodiment of the present invention;

FIG. 14 is an explanatory diagram that shows the dot recording method ofthe second comparative example;

FIG. 15 is an explanatory diagram that shows the dot recording method ofthe fourth embodiment of the present invention;

FIG. 16 is an explanatory diagram that shows the dot recording method ofthe fifth embodiment of the present invention;

FIG. 17 is an explanatory diagram that shows the dot recording method(relationship between the passes and rasters) of the sixth embodiment ofthe present invention;

FIG. 18 is an explanatory diagram that shows the dot recording method(relationship between the rasters and nozzles) of the sixth embodimentof the present invention;

FIG. 19 is an explanatory diagram that shows the dot recording method ofthe seventh embodiment of the present invention;

FIG. 20 is an explanatory diagram that shows a case when the dotposition has shifted in the raster direction (main scanning direction);

FIG. 21 is a graph that shows the relationship between the spacefrequency for human visual characteristics and the number ofidentifiable tones;

FIG. 22 is an explanatory diagram that shows the dot recording method ofthe eighth embodiment of the present invention;

FIG. 23 is an explanatory diagram that shows the dot recording method ofthe ninth embodiment of the present invention;

FIG. 24 is an explanatory diagram that shows the dot recording method ofthe tenth embodiment of the present invention;

FIG. 25 is an explanatory diagram that shows the dot recording method(pass 1 to pass 10) of the eleventh embodiment of the present invention;

FIG. 26 is an explanatory diagram that shows the dot recording method(pass 2 to pass 12) of the eleventh embodiment of the present invention;

FIG. 27 is an explanatory diagram that shows the dot recording method(pass 3 to pass 13) of the eleventh embodiment of the present invention;

FIG. 28 is an explanatory diagram that shows the dot recording method(pass 1 to pass 9) of the twelfth embodiment of the present invention;

FIG. 29 is an explanatory diagram that shows the dot recording method(pass 2 to pass 11) of the twelfth embodiment of the present invention;

FIG. 30 is an explanatory diagram that shows the dot recording method(pass 4 to pass 12) of the twelfth embodiment of the present invention;

FIG. 31(A) is an explanatory diagram that shows sub-scan feed for aninterlace recording method;

FIGS. 31(B) and 31(C) show the numbers of the nozzles that record dotson each main scan line;

FIG. 32(A) is an explanatory diagram that shows sub-scan feed for anoverlapping recording method;

FIGS. 32(B) and 32(C) show the numbers of the nozzles that record dotson each main scan line;

FIG. 33 is an explanatory diagram that shows the dot recording method ofthe thirteenth embodiment of the present invention;

FIG. 34 is an explanatory diagram that shows the dot recording method(pass 1 to pass 14) of the fourteenth embodiment of the presentinvention;

FIG. 35 is an explanatory diagram that shows the dot recording method(pass 4 to pass 17) of the fourteenth embodiment of the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention is explained in the following sequence based onembodiments.

A. Apparatus Structure:

B. Basic Conditions of the Recording Method:

C. Concept of Main scanning for Intermittent Overlapping Method

D. Constant Feed Dot Recording Method Comparative Example andEmbodiments:

E. Variable Feed Dot Recording Method Comparative Example andEmbodiments:

F. Embodiment of Dot Recording Method That Removes Low Frequency ColorIrregularity:

G. Embodiment of Dot Recording Method with Variable Number of Mainscanning:

H. Variation example:

A. Apparatus Structure

FIG. 1 is a block diagram that shows the structure of a printing systemas an embodiment of the present invention. This printing system has acomputer 90 as a printing control apparatus, and a color printer 20 as aprinting unit. The combination of color printer 20 and computer 90 canbe called a “printing apparatus” in its broad definition.

Application program 95 operates on computer 90 under a specificoperating system. Video driver 91 and printer driver 96 are incorporatedin the operating system, and print data PD to be sent to color printer20 is output via these drivers from application program 95. Applicationprogram 95 performs the desired processing on the image to be processed,and displays the image on CRT 21 with the aid of video driver 91.

When application program 95 issues a print command, printer driver 96 ofcomputer 90 receives image data from application program 95, andconverts this to print data PD to supply to color printer 20. In theexample shown in FIG. 1, printer driver 96 includes resolutionconversion module 97, color conversion module 98, Halftone module 99,rasterizer 100, and color conversion table LUT.

Resolution conversion module 97 has the role of converting theresolution (in other words, the pixel count per unit length) of thecolor image data handled by application program 95 to resolution thatcan be handled by printer driver 96. Image data that has undergoneresolution conversion in this way is still image information made fromthe three colors RGB. Color conversion module 98 converts RGB image datato multi-tone data of multiple ink colors that can be used by colorprinter 20 for each pixel while referencing color conversion table LUT.

The color converted multi-tone data can have a tone value of 256 levels,for example. Halftone module 99 executes halftone processing to expressthis tone value on color printer 20 by distributing and forming inkdots. Image data that has undergone halftone processing is realigned inthe data sequence in which it should be sent to color printer 20 byrasterizer 100, and ultimately is output as print data PD. Print data PDincludes raster data that shows the dot recording state during each mainscan and data that shows the sub-scan feed amount.

Printer driver 96 is a program for realizing a function that generatesprint data PD. A program for realizing the functions of printer driver96 is supplied in a format recorded on a recording medium that can beread by a computer. As this kind of recording medium, any variety ofcomputer readable medium can be used, including floppy disks, CD-ROMs,opt-magnetic disks, IC cards, ROM cartridges, punch cards, printed itemson which a code such a bar code is printed, a computer internal memorydevice (memory such as RAM or ROM), or external memory device, etc.

FIG. 2 is a schematic structural diagram of color printer 20. Colorprinter 20 is equipped with a sub-scan feed mechanism that carriesprinting paper P in the sub-scanning direction using paper feed motor22, a main scan feed mechanism that sends cartridge 30 back and forth inthe axial direction of platen 26 using carriage motor 24, a head drivingmechanism that drives printing head unit 60 built into carriage 30 andcontrols ink ejecting and dot formation, and control circuit 40 thatcontrols the interaction between the signals of paper feed motor 22,carriage motor 24, printing head unit 60, and operating panel 32.Control circuit 40 is connected to computer 90 via connector 56.

The sub-scan feed mechanism that carries printing paper P is equippedwith a gear train (not illustrated) that transmits the rotation of paperfeed motor 22 to paper carriage roller (not illustrated). Also, the mainscan feed mechanism that sends carriage 30 back and forth is equippedwith sliding axis 34 on which is supported carriage 30 so that it canslide on the axis and that is constructed in parallel with the axis ofplaten 26, pulley 38 on which is stretched seamless drive belt 36between the pulley and carriage motor 24, and position sensor 39 thatdetects the starting position of carriage 30.

FIG. 3 is a block diagram that shows the structure of color printer 20,the core of which is control circuit 40. Control circuit 40 is formed asan arithmetic and logic operating circuit that is equipped with CPU 41,programmable ROM (PROM) 43, RAM 44, and character generator (CG) 45 thatstores the dot matrix of the characters. This control circuit 40 isfurther equipped with an interface circuit 50 that works exclusively asan interface with external motors, etc., head drive circuit 52 connectedto this interface circuit 50 that drives printing head unit 60 andejects ink, motor drive circuit 54 that drives paper feed motor 22 andcarriage motor 24, and scanner control circuit 55 that controls scanner80. Interface circuit 50 has a built in parallel interface circuit, andcan receive print data PD supplied from computer 90 via connector 56.Color printer 20 executes printing according to this print data PD. RAM44 functions as buffer memory for temporarily storing raster data.

Printing head unit 60 has printing head 28, and holds an ink cartridge.Printing head unit 60 can be attached and detached from color printer 20as a part. In other words, printing head 28 is replaced together withprinting head unit 60.

FIG. 4 is an explanatory diagram that shows the nozzle array on thebottom surface of printing head 28. Formed on the bottom surface ofprinting bead 28 are black ink nozzle group K_(D) for ejecting blackink, dark cyan ink nozzle group C_(D) for ejecting dark cyan ink, lightcyan ink nozzle group C_(L), for ejecting light cyan ink, dark magentaink nozzle group M_(D) for ejecting dark magenta ink, light magenta inknozzle group M_(L) for ejecting light magenta ink, and yellow ink nozzlegroup Y_(D) for ejecting yellow ink.

The upper case alphabet letters at the beginning of the referencesymbols indicating each nozzle group means the ink color, and thesubscript “D” means that the ink has a relatively high density and thesubscript “L” means that the ink has a relatively low density.

The multiple nozzles of each nozzle group are each aligned at a fixednozzle pitch k·D along sub-scanning direction SS. Here, k is an integer,and D is the pitch (called “dot pitch”) that correlates to the printingresolution in the sub-scanning direction. In this specification, we alsosay “the nozzle pitch is k dots.” The “dot” unit means the printresolution dot pitch. Similarly, the “dot” unit is used for sub-scanfeed amount as well.

Each nozzle is provided with a piezoelectric element (not illustrated)as a drive component that drives each nozzle to ejects ink drops. Inkdrops are ejected from each nozzle while printing head 28 is moving inmain scan direction MS.

Multiple nozzles of each nozzle group do not have to be arrayed in astraight line along the sub-scanning direction, but can also be arrayedin a zigzag, for example. Even when the nozzles are arrayed in a zigzag,the nozzle pitch k·D measured in the sub-scanning direction can bedefined in the same way as the case shown in FIG. 4. In thisspecification, the phrase “multiple nozzles arrayed along thesub-scanning direction” has a broad meaning that includes nozzlesarrayed in a zigzag.

Color printer 20 that has the hardware configuration described above,while carrying paper P using paper feed motor 22, sends carriage 30 backand forth using carriage motor 24, and at the same time drives thepiezoelectric element of printing head 28, ejects ink drops of eachcolor to form ink drops and forms a multi-tone image on paper P.

B. Basic Conditions of the Recording Method

Before giving a detailed explanation of the recording method used in theembodiments of the present invention, first, the basic conditions of anormal interlace recording method is explained hereafter. An “interlacerecording method” means a recording method that is used when the nozzlepitch k in the sub-scanning direction is two or greater. With aninterlace recording method, with one main scan, a raster line thatcannot be recorded is left between adjacent nozzles, and the pixels onthis raster line are recorded during another main scan. In thisspecification, “printing method” and “recording method” are synonyms.

FIG. 5(A) shows an example of sub-scan feed of an ordinary interlacerecording method, and FIG. 5(B) shows its parameters. In FIG. 5 (A), thesolid line circle around the numbers indicates positions of the fournozzles in the sub-scanning direction for each pass. The term “pass”means one main scan. The numbers 0 through 3 in the circles indicate thenozzle numbers. The positions of the four nozzles shift in thesub-scanning direction each time one main scan ends. However, inreality, the sub-scanning direction feed is realized by movement of thepaper by paper feed motor 22 (FIG. 2).

As shown at the left side of FIG. 5(A), with this example, sub-scan feedamount L is a fixed value of four dots. Therefore, each time a sub-scanfeed is done, the position of the four nozzles shifts by four dots eachin the sub-scanning direction. Each nozzle has as a recording target alldot positions (also called “pixel positions”) on each raster line duringone main scan. In this specification, the total number of main scansperformed on each raster line (also called “main scan lines”) is called“scan repetition count s.”

At the right side of FIG. 5(A) is shown the ordinal number of the nozzlethat records dots on each raster line. With the raster lines drawn by adotted line extending in the right direction (main scan direction) fromthe circles that indicate the sub-scanning direction position of thenozzles, at least one of the raster lines above or below this cannot berecorded, so in fact, dot recording is prohibited. Meanwhile, the rasterlines drawn by a solid line extending in the main scan direction are ina range for which dots can be recorded on the raster lines before andafter them. The range for which recording can actually be done willhereafter be called the valid recording range (or “valid printingrange,” “printing execution area,” or “recording execution area”).

In FIG. 5(B), various parameters relating to this dot recording methodare shown. Dot recording method parameters include nozzle pitch k(dots), the number of working nozzles N, the scan repetition count s,the effective nozzle count Neff, and sub-scan feed amount L (dots).

In the example in FIGS. 5(A) and 5(B), nozzle pitch k is 3 dots. Numberof working nozzles N is 4. Also, number of working nozzles N is thenumber of nozzles actually used among the multiple nozzles that areinstalled. Scan repetition count s means that main scans are executed stimes on each raster line. For example, when scan repetition count s is2, main scans are executed twice on each raster line. At this time,normally dots are formed intermittently at every other dot position onone main scan. In the case shown in FIGS. 5(A) and 5(B), the scanrepetition count s is 1. The effective nozzle count Neff is a value ofworking nozzle number N divided by scan repetition count s. Thiseffective nozzle count Neff can be thought of as showing the net numberof the raster lines for which dot recording is completed with one mainscan.

In the table in FIG. 5(B), the sub-scan feed amount L, its sum value ΣL,and nozzle offset F are shown for each pass. Here, offset F indicateshow many dots the nozzle position is separated in the sub-scanningdirection from the reference positions for each pass; the referencepositions for which the offset is 0 are cyclical positions of thenozzles (in FIGS. 5(A) and 5(B), a position every three dots) at thefirst pass. For example, as shown in FIG. 5(A), after pass 1, the nozzleposition moves in the sub-scanning direction by sub-scan feed amount L(4 dots). Meanwhile, nozzle pitch k is 3 dots. Therefore, the nozzleoffset F for pass 2 is 1 (see FIG. 5(A)). Similarly, the nozzle positionfor pass 3 is moved from the initial position by ΣL=8 dots, and theoffset F is 2. The nozzle position for pass 4 moves ΣL=12 dots from theinitial position, and the offset F is 0. With pass 4 after threesub-scan feeds, nozzle offset F returns to 0, and by repeating a cycleof three sub-scans, it is possible to record dots on all raster lines inthe valid recording range.

As can be understood from the example in FIGS. 5(A) and 5(B), when thenozzle position is in a position separated by an integral multiple ofnozzle pitch k from the initial position, offset F is 0. In addition,offset F can be given by remainder (ΣL) % k, which is obtained bydividing cumulative value ΣL of sub-scan feed amount L by nozzle pitchk. Here, “%” is an operator that indicates that the division remainderis taken. If we think of the nozzle initial position as a cyclicalposition, we can also think of offset F as showing the phase shiftamount from the initial position of the nozzle.

When the scan repetition count s is 1, to have no gaps or overlap in theraster line that is to be recorded in the valid recording range, thefollowing conditions must be met.

Condition c1: The number of sub-scan feeds of one cycle is equal tonozzle pitch k.

Condition c2: Nozzle offset F after each sub-scan feed in one cycleassumes a different value in a range from 0 to (k−1).

Condition c3: The average sub-scan feed amount (ΣL/k) is equal to theworking nozzle number N. In other words, the cumulative value ΣL ofsub-scan feed amount L per cycle is equal to the working nozzle number Nmultiplied by nozzle pitch k, (N×k).

Each of the aforementioned conditions can be understood by thinking asfollows. There are (k−1) raster lines between adjacent nozzles. In orderfor a nozzle to return to the reference position (position where offsetF is 0) while performing recording on these (k−1) raster lines duringone cycle, the number of sub-scan feeds in one cycle will be k. If thenumber of sub-scan feeds in one cycle is less than k, there will be gapsin the recorded raster lines, and if there are more than k sub-scanfeeds in one cycle, there will be overlap in the recorded raster lines.Therefore, the aforementioned first condition c1 is established.

When the number of sub-scan feeds in one cycle is k, gaps and overlapsin the recorded raster lines are eliminated only when the values ofoffset F after each sub-scan feed are different from each other in therange 0 to (k−1). Therefore, the aforementioned second condition c2 isestablished.

If the aforementioned first and second conditions are established,during one cycle, recording of k raster lines will be performed for eachof N nozzles. Therefore, with one cycle, recording of N×k raster linesis performed. Meanwhile, if the aforementioned third condition c3 ismet, as shown in FIG. 5(A), the nozzle position after one cycle (after ksub-scan feeds) comes to a position separated by N×k raster lines fromthe initial nozzle position. Therefore, by fulfilling the aforementionedfirst through third conditions c1 to c3, it is possible to eliminategaps and overlaps in the range of these N×k raster lines.

FIGS. 6(A) and 6(B) show the basic conditions of a dot recording methodwhen the scan repetition count s is 2. Hereafter, we will call a dotrecording method for which the scan repetition count s is 2 or greateran “overlapping method”. FIG. 6(A) shows an example of sub-scan feed ofthe overlapping interlace recording method, and FIG. 6(B) shows itsparameters. When the scan repetition count s is 2 or greater, mainscanning is executed s times on the same raster line.

The dot recording method shown in FIGS. 6(A) and 6(B) has a differentscan repetition count s and sub-scan feed amount L for the parameters ofthe dot recording method shown in FIG. 5 (B). As can be seen from FIG.6(A), the sub-scan feed amount L of the dot recording method in FIGS.6(A) and 6(B) is a fixed value of 2 dots. In FIG. 6(A), the positions ofnozzles at even numbered passes are shown by a diamond shape. Normally,as shown at the right side of FIG. 6(A), the recorded dot positions oneven numbered passes are shifted by one dot in the main scan directionfrom those on the odd numbered passes. Therefore, multiple dots on thesame raster line are intermittently recorded by two different nozzles.For example, the topmost raster line within the valid recording range isintermittently recorded every other dot by the #0 nozzle on pass 5 afterintermittent recording is done every other dot by the #2 nozzle on pass2. With this overlapping method, each nozzle is driven with intermittenttiming so that (s−1) dot recording is prohibited after 1 dot is recordedduring one main scan.

In this way, the overlapping method that has intermittent pixelpositions on a raster line as a recording target during each main scanis called an “intermittent overlapping method.” Also, instead of havingintermittent pixel positions as the recording target, it is alsopossible to have all pixel positions on a raster line during each mainscan be the recording target. In other words, when executing a main scans times on one raster line, it is allowable to overstrike dots on thesame pixel position. This kind of overlapping method is called an“overstrike overlapping method” or “complete overlapping method”.

With an intermittent overlapping method, it is acceptable, as far as thetarget pixel positions of the multiple nozzles on the same raster lineare shifted in relation to each other, so for the actual shift amount inthe main scan direction during each main scan, a variety of shiftamounts other than that shown in FIG. 6(A) are possible. For example, itis also possible to record dots in the positions shown by circleswithout shifting in the main scan direction on pass 2, and to record thedots in the positions shown by diamonds with the shift in the main scandirection performed on pass 5.

The value of offset F of each pass in one cycle is shown at the bottomof the table in FIG. 6(B). One cycle includes six passes, and offset Ffor pass 2 to pass 7 includes a value in the range of 0 to 2 twice each.Also, the change in offset F for three passes from pass 2 to pass 4 isequal to the change in offset F for three passes from pass 5 to pass 7.As shown at the left side of FIG. 6(A), the six passes of one cycle canbe segmented into two small cycles of three passes each. At this time,one cycle ends by repeating a small cycle s times.

Generally, when scan repetition count s is an integer of 2 or greater,the first through third conditions c1 through c3 described above can berewritten as the following conditions c1′ through c3′.

Condition c1′: The sub-scan feed count of one cycle is equal to themultiplied value of nozzle pitch k and scan repetition count s, (k×s).

Condition c2′: Nozzle offset F after each of the sub-scan feeds in onecycle assumes a value in the range of 0 through (k−1), and each value isrepeated s times.

Condition c3′: The sub-scan average feed amount {ΣL/(k×s)} is equal toeffective nozzle count Neff (=N/s). In other words, cumulative value ΣLof sub-scan feed amount L per cycle is equal to the multiplied value ofeffective nozzle count Neff and the sub-scan feed count (k×s),{Neff×(k×s)}.

The aforementioned conditions c1′ through c3′ also holds when scanrepetition count s is 1. Therefore, conditions c1′ to c3′ can be thoughtof as conditions that are generally established in interlace recordingmethods regardless of the value of scan repetition count s. In otherwords, if the aforementioned three conditions c1′ through c3′ aresatisfied, it is possible to eliminate gaps and unnecessary overlaps forrecorded dots in the valid recording range. However, when using theintermittent overlapping method, a condition is required whereby therecording positions of nozzles that record on the same raster line areshifted in relation to each other in the main scan direction. Inaddition, when using an overstrike overlapping method, it is enough tosatisfy the aforementioned conditions c1′ to c3′, and for each pass, allpixel positions are subject to recording.

In FIGS. 5(A), 5(B), 6(A), and 6(B), cases when sub-scan feed amount Lis a fixed value are explained, but the aforementioned conditions c1′ toc3′ can be applied not only in cases when sub-scan feed amount L is afixed value, but also in cases of using a combination of multipledifferent values as the sub-scan feed amount. Note that in thisspecification, sub-scan feeds for which feed amount L is a fixed valueare called “constant feeds,” and sub-scan feeds that use combinations ofmultiple different values as the feed amount are called “variablefeeds.”

C. Concept of Main Scanning for Intermittent Overlapping Method

FIG. 7 is a block diagram that shows the main configuration of headdrive circuit 52 (FIG. 3). Head drive circuit 52 is equipped with drivesignal generator 220, masking circuits 222, and piezoelectric element PEof each nozzle. Masking circuits 222 are provided for each nozzle #1,#2, . . . of printing head 28. In addition, in FIG. 7, the number inparentheses added at the end of the signal names show the ordinal numberof the nozzle to which that signal is supplied.

FIG. 8(A) is a timing chart that shows the operation of head drivecircuit 52 for a non-overlapping interlace method. Drive signalgenerator 220 generates the original drive signal COMDRV used in commonby each nozzle and supplies this to masking circuits 222. This originaldrive signal COMDRV is a signal that includes one pulse in one pixelperiod Td. The i-th masking circuit 222 masks original drive signalCOMDRV according to the level of serial printing signal PRT (i) of thei-th nozzle. Specifically, masking circuits 222 pass original drivesignal COMDRV as is when printing signal PRT (i) is level 1. To supplyit to piezoelectric element PE as drive signal DRV. Meanwhile, when theprinting signal PRT (i) is level 0, original drive signal COMDRV isblocked. This serial printing signal PRT (i) indicates the recordingstate of each pixel during one main scan by the i-th nozzle. This signalPRT(i) is derived from print data PD (FIG. 1) given from computer 90.FIG. 8(A) shows an example of when dots are recorded every other pixel.When dots are recorded for all pixels, original drive signal COMDRV issupplied as is to piezoelectric element PE as drive signal DRV.

FIG. 8(B) is a timing chart when dots are formed at odd numbered pixelpositions using an intermittent overlapping method for which the scanrepetition count s is 2, and FIG. 8(C) is a timing chart when dots areformed at even numbered pixel positions. With these examples, thewaveform of the original drive signal COMDRV is generated at a rate ofone pixel in two. Therefore, when the original drive signal waveform ofFIG. 8(B) is used, even in a case when all serial printing signal PRT(i) are “1,” dots can be formed only at the odd numbered pixelpositions. Similarly, when the original drive signal waveform of FIG.8(C) is used, even in a case when all serial printing signals PRT (i)are “1,” dots can be formed only at the even numbered pixel positions.The reason that for the intermittent overlapping method the originaldrive signal COMDRV appears only in intermittent pixel positions in thisway is to increase printing speed as explained hereafter.

Generally, with the condition of having the main scan speed being thesame, printing speed is proportional to effective nozzle count Neff (inother words, the number of main scan lines for which dot formation iscompleted with one main scan). As described above, effective nozzlecount Neff is a value of used nozzle count N divided by scan repetitioncount s. Therefore, with the condition that the main scan speed and usednozzle count are the same, printing speed is inversely proportional toscan repetition count s. For example, the overlapping method shown inFigure has a printing speed that is ½ that of the non-overlapping methodshown in FIGS. 5(A) and 5(B).

In this way, when an overlapping method is used, the printing speeddecreases. However, if the main scan speed is increased, it is possibleto soften the degree of the printing speed reduction. For example, whenscan repetition count s is 2, if the main scan speed is doubled, thenthe printing speed is the same as when scan repetition count s is 1.However, typically, the upper limit of the nozzle drive frequency(number of ink ejects per time unit) limits the main scan speed. Inother words, to form dots at suitable pixel positions, it is alsonecessary to increase the nozzle drive frequency according to theincrease in main scan speed. However, when the nozzle drive frequency isincreased excessively, it is not possible to eject a suitable amount ofink. Therefore, to eject a suitable amount of ink at suitable pixelpositions, there is an upper limit to the nozzle drive frequency, andthus there is a limit to the main scan speed as well.

In this way, the fact that there is an upper limit to the nozzle drivefrequency limits the main scan speed. However, if ink ejecting isintermittent in the main scan direction, it is possible to make the mainscan speed faster. For example, when ink is ejected intermittently inthe main scan direction at a rate of one pixel in two, if the main scanspeed is the same, the nozzle drive frequency will be sufficient athalf. Generally, if ink is ejected at a rate of one pixel in q, even ifthe main scan speed is raised by q times, the nozzle drive frequencydoes not change, and ink can reach desired positions in the main scandirection.

D. Constant Feed Dot Recording Method (Comparative Example andEmbodiments)

FIG. 9 is an explanatory diagram that shows a first comparative example,which is a constant feed overlapping method. The parameters of thisrecording method are N=6, k=4, L=3, and s=2. These parameters fulfillconditions c1′ through c3′ described above. Therefore, it is possible toexecute printing without gaps or unnecessary overlaps for the recordeddots.

The pixel position numbers shown at the right side of FIG. 9 show thesequence of the pixels on each raster line, and the numbers in circlesindicate numbers of nozzles in charge of forming dots at those pixelpositions. For example, the first raster line has dots formedalternately by the #1 and #4 nozzles. In other words, it shows that onthe first raster line, the dot of pixel position #1 is formed by nozzle#4, and the dot of pixel position #2 is formed by nozzle #1. Similarly,the dots on the second raster line are formed by nozzles #2 and #5, andthe dots on the third raster line are formed by nozzles #3 and #6. Then,generally, the (1+3×n)th raster line is formed by nozzles #1 and #4, the(2+3×n)th raster line is formed by nozzles #2 and #5, and the (3+3×n)thraster line is formed by nozzles #3 and #6. Meanwhile, when we look atpasses, the (1+8×n)th, (2+8×n)th, (3+8×n)th, and (4+8×n)th passes formdots only at odd numbered pixel positions, and the (5+4×n), (6+4×n),(7+4×n), and (8+4×n) numbered passes form dots only at even numberedpixel positions.

For this first comparative example, the recording target pixel ratio foreach nozzle is 0.5. Here, the “recording target pixel ratio” of aparticular nozzle means the proportion of pixels for which dots areformed when that nozzle passes over one raster line. For this firstcomparative example, all of the working nozzles have the pixels on eachraster line as targets of dot formation at a rate of one pixel in two.Therefore, the recording target pixel ratio for all nozzles is 0.5. Inthe embodiment described later, the recording target pixel ratio isdifferent for each nozzle. However, typically, from that definition, thesum of the recording target pixel ratios relating to multiple nozzles incharge of forming dots on one raster line becomes 1.0.

In addition, the intermittence level q is 2 in this first embodiment.Here, intermittence level q means the value of the total number ofpixels of one raster line divided by the number of pixels for which onenozzle can form dots on one pass. In the first comparative example, dotscan only be formed in the even numbered rows or odd numbered rows forall of the passes. Therefore, dots can be formed at half the pixelpositions of the raster lines on each pass, and the intermittence levelq is 2. In addition, intermittence level q is closely related to themain scan speed. Specifically, if intermittence level q is increased,the dot formation frequency for a main scan decreases, so it is possibleto reduce the nozzle drive frequency and thus to increase the main scanspeed.

FIG. 10 is an explanatory diagram that shows the dot recording method ofthe first embodiment of the present invention. This dot recording methodis different from the first comparative example shown in FIG. 9 in thatnozzle #7 is added to the working nozzles. This nozzle #7 forms dots on(1+3×n)th raster line together with nozzles #1 and #4. In the firstembodiment, two nozzles, #1 and #7, form dots on even numbered pixelposition alternately while, in the first comparative example, nozzle #1alone forms dots on the even numbered pixel positions. As to the oddnumbered pixel positions, in both the first comparative example and thefirst embodiment, nozzle #4 forms the dots. Meanwhile, on the (2+3×n)thand (3+3×n)th raster lines, as with the first comparative example, inthe first embodiment as well, dots are formed by two nozzles. With thisspecification, the raster lines recorded by multiple nozzles are called“overlapping raster lines.” In the first embodiment, the (1+3×n)thraster line is recorded by three nozzles, but the (2+3×n)th and(3+3×n)th raster lines are recorded by two nozzles. In other words, thenumber of overlaps (number of working nozzles per raster line) differsfor each raster line. This point is the difference between the firstembodiment and the first comparative example.

As described above, in the first comparative example of FIG. 9, therecording target pixel ratio for each nozzle is 0.5. Meanwhile, for thefirst embodiment, the recording target pixel ratio for nozzle #4 is 0.5,but that for nozzles #1 and #7 is 0.25. In other words, nozzles #1 and#7 have pixel positions at a rate of one pixel in four as the recordingtarget.

Generally, the sum of the recording target pixel ratios for multiplenozzles in charge of dot formation on each raster line is 1.0. Forexample, in the first comparative example in FIG. 9, the recordingtarget pixel ratio for both of the two nozzles #1 and #4 in charge ofrecording dots on the first raster line is 0.5, and the sum of these is1.0. Therefore, it is possible to complete a raster line without gaps inthe pixel positions that are targets of recording. Meanwhile, in thefirst embodiment, dot formation is performed on the first raster line bynozzle #4 for which the recording target pixel ratio is 0.5 and bynozzles #1 and #7 for which the recording target pixel ratio is 0.25. Inthis case as well, the sum of the recording target pixel ratios is 1.0,and we can see that the raster line can be completed without any gaps.

FIG. 11(A) shows the allocation of raster data to each nozzle for thefirst comparative example of FIG. 9. The values of the raster data thatshows the dot formation state on the first raster line are 1, 1, 1, 0,0, 1 . . . Here, the value “1” shows that a dot is recorded at thatpixel position, and the value “0” means that a dot is not recorded. Withthis first comparative example, for the first raster line, nozzle #1 isin charge of recording even numbered pixels, and nozzle #4 is in chargeof recording odd numbered pixels. Also, there is no odd numbered pixelposition data in the raster data allocated to nozzle #1, and only evennumbered pixel position data is arrayed consecutively. As shown in FIG.8(B) described above, this is because when dots are formed at oddnumbered positions with an overlapping method, recording is not possibleat even numbered pixel positions, so even numbered pixel position datais omitted in advance. Similarly, there is no even numbered pixelposition data in the raster data allocated to nozzle #4, and only oddnumbered pixel position data is arrayed consecutively. For the secondraster line, nozzle #2 is in charge of recording even numbered pixels,and nozzle #5 is in charge of recording odd numbered pixels. Then, forthe third raster line, nozzle #3 is in charge of recording even numberedpixels, and nozzle #6 is in charge of recording odd numbered pixels.Raster data allocated to each nozzle in this way correspond to serialprinting signal PRT (i) shown in FIGS. 8(B) and 8(C).

FIG. 11(B) shows the allocation of raster data to each nozzle in thefirst embodiment of FIG. 10. For the second and third raster lines, thedata is the same as for the first comparative example shown in FIG.11(A), and the first raster line data is different from that of thefirst comparative example.

With the first embodiment, for the first raster line, the (4+4×n)thpixel position raster data is allocated to nozzle #1, the (2+4×n)thpixel position raster data is allocated to nozzle #7, and the oddnumbered pixel position raster data is allocated to nozzle #4. Further,for the raster data allocated to nozzles #4 and #7, dummy data isallocated to the pixel positions for which those nozzles are not incharge of dot recording. Here, “dummy data” is data for which the value“0” is allocated regardless of the original raster data value. As aresult, it is possible to have the target of dot recording be the evennumbered pixel positions on the first raster line without gaps oroverlap using two nozzles #1 and #7.

Control circuit 40 (FIG. 2) carries the printing medium in the main scandirection by L dots each time one main scan ends, and as a result,printing head 28 moves to the position of pass 2 from pass 1 in FIG. 10,for example. Nozzle #7 is positioned on the first raster line on pass 1,nozzle #4 on pass 5, and nozzle #1 on pass 9. In light of this, nozzles#7, #4, and #1 record designated pixels on these raster lines accordingto the raster data shown in FIG. 11(B). As a result, complementaryrecording operations are completed on the first raster line. Byrepeating the above operation, text and images are formed on theprinting medium.

In the first embodiment in FIG. 10, three nozzles are in charge of dotformation on some raster lines. Therefore, it is possible to reducebanding (degradation of the image in a stripe shape extending in themain scan direction) compared to that of the first comparative exampleof FIG. 9. Also, raster lines for which dot recording is performed bythree nozzles appear once each cycle for every three lines. In otherwords, there is variation in the number of nozzles that perform dotforming on one raster line, and this also has the effect of reducingbanding.

From the meaning of having banding be less prominent, we can alsoconsider increasing the number of nozzles in charge of dot recording forall raster lines. However, when the number of working nozzles for allthe raster lines are increased uniformly, there is an excessivereduction in printing speed. In comparison to this, with the firstembodiment described above, the number of working nozzles is mixed withdifferent raster lines, so compared to the case of setting the number ofworking nozzles for all raster lines uniformly, there is the advantageof having it easier to attain a good balance between image quality andprinting speed.

With bi-directional printing whereby main scanning is done in bothdirections, the above described dot recording also exhibits the effectdescribed below. Specifically, as shown in FIG. 4, when a nozzle arrayof six colors of ink of Y_(D), M_(D), M_(L), C_(D), C_(L), and K_(D) areplaced to record the same raster line, on the outgoing pass, eachcolored dot is formed on each raster line in the sequence K_(D), C_(D),C_(L), M_(D), M_(L), and Y_(D). Meanwhile, on the return pass,conversely, each colored dot is formed on each raster line in thesequence Y_(D), M_(L), M_(D), C_(L), C_(D), and K_(D). Therefore, it ispossible to see a slight color difference between the raster linerecorded on the outgoing pass and the raster line recorded on the returnpass. At this time, when the conventional interlace recording method isused to record dots rather than using an overlapping method, thedifference in colors between the raster line recorded on the outgoingpass and that recorded on the return pass may be quite noticeable. Thisis recognized as image degradation. Thus, as with the aforementionedembodiment, where an overlapping method is used, there is the advantagethat the difference in colors of the raster lines with the outgoing passand return pass is not so noticeable.

FIG. 12 is an explanatory diagram that shows the recording method of thesecond embodiment of the present invention. The difference from thefirst comparative example shown in FIG. 10 is that nozzle #10 is furtheradded to the working nozzles. However, nozzles #8 and #9 are not used.The (1+3×n)th raster line is recorded by the four nozzles #1, #4, #7,and #10. These nozzles #1, #4, #7, and #10 have pixel positions asrecording targets at a rate of one pixel in four, so the recordingtarget pixel ratio is 0.25. As with the first embodiment, the (2+3×n)thraster line and (3+3×n)th raster line are recorded by two nozzles each.

For the second embodiment, raster lines for which dots are recorded bytwo nozzles and raster lines for which dots are recorded by four nozzlesare mixed. Therefore, compared with the first comparative example ofFIG. 9 where all raster lines are recorded by two nozzles, it ispossible to reduce the banding. Also, raster lines for which dotrecording is performed by four nozzles appear once each cycle everythree lines. In other words, there is variation in the number of nozzlesthat perform dot formation of on one raster line, and this also has theeffect of reducing banding.

FIG. 13 is an explanatory diagram that shows the recording method of thethird embodiment of the present invention. The difference with thesecond embodiment shown in FIG. 12 is that two nozzles #8 and #9 arefurther added to the working nozzles. Dot formation on the first rasterline recording is the same as that of the first embodiment, but that onthe second and third raster lines is different. For the second rasterline, nozzle #8 is in charge of dot recording for the (1+4×n)th pixelposition, and nozzle #5 is in charge of even numbered pixel position dotrecording, and nozzle #2 is in charge of dot recording for (3+4×n)thpixel positions. The recording target pixel ratio for nozzles #8 and #2is 0.25, and the ratio for nozzle #5 is 0.5. Dot recording for the thirdraster line is completed by three nozzles #3, #6, and #9.

As can be seen from embodiments 1 through 3 described above, by suitablyadding some suitable nozzles to the working nozzles in the firstcomparative example of FIG. 9 where uniform overlapping method is usedwith a scan repetition count s of 2, it is possible to increase thenumber of nozzles in charge of dot recording on several raster lines to3 or 4. As a result, it is possible to reduce banding when compared to auniform overlapping method. It is also possible to set the main scanspeed and sub-scan feed amount of these embodiments to the same as thoseof the first comparative example, so the banding can be reduced withoutreducing the printing speed.

E. Variable Feed Dot Recording Method (Comparative Example andEmbodiments)

FIG. 14 is an explanatory diagram that shows the second comparativeexample that is a variable feed uniform overlapping method. Theparameters of this recording method are N=12, k=4, and s=2, and assub-scan feed amount L, 6 dots, 5 dots, 6 dots, and 7 dots arerepeatedly used. These parameters satisfy conditions c1′ through c3′described above. Therefore, it is possible to execute printing withoutgaps or unnecessary overlap of the recorded dots. In addition, allraster lines are recorded by two nozzles.

For this second comparative example as well, as with the firstcomparative example (FIG. 9) described above, only even numbered pixelposition raster data is allocated to nozzles that record even numberedpixel positions, and only odd numbered pixel position raster data isallocated to nozzles that record odd numbered pixel positions.

FIG. 15 is an explanatory diagram that shows the recording method of afourth embodiment of the present invention. For this fourth embodiment,nozzles #13 and #19 are added to the working nozzles for the secondcomparative example. Note that nozzles #14 through #18 are not used.

In the fourth embodiment, raster lines for which dots are recorded usingtwo nozzles and raster lines for which dots are recorded using fournozzles are mixed together. Therefore, compared to case where all rasterlines are recorded using two nozzles as in the second comparativeexample, it is possible to reduce banding. In addition, the raster linesfor which dot recording is performed with four nozzles appear once everysix lines.

In the fourth embodiment as well, the raster data to each nozzle is thesame as in FIG. 11(B). Specifically, even numbered pixel position datais allocated to nozzles #1 and #13 which are in charge of recording evennumbered pixel positions and for which the recording pixel ratio is0.25, and dummy data is allocated to positions in the even numberedpositions for which those nozzles are not in charge of recording.Similarly, odd numbered pixel position data is allocated to nozzles #7and #19 which are in charge of recording odd numbered pixel positionsand for which the recording pixel ratio is 0.25, and dummy data isallocated to positions in the odd numbered positions for which thosenozzles are not in charge of recording.

FIG. 16 is an explanatory diagram that shows the recording method of afifth embodiment of the present invention. The difference between thisand the fourth embodiment shown in FIG. 15 is that nozzles #14 through#18 are added to the working nozzles. Consequently, the raster linesrecorded by two nozzles in FIG. 14 are recorded by three nozzles in FIG.15.

As can be seen from the fourth and fifth embodiments described above,even when variable feed is used, by adding some suitable nozzles to theworking nozzles in a uniform overlapping method such as the secondcomparative example, it is possible to increase the number nozzles incharge of dot recording on some raster lines 3 or 4. As a result, it ispossible to decrease the banding compared to that of a uniformoverlapping method. Also, with these embodiments, for the main scanspeed, it is possible to set the sub-scan feed amount to the same as forthe second comparative example, so banding can be reduced withoutdecreasing the printing speed.

FIG. 17 is an explanatory diagram that shows the recording method of thesixth embodiment of the present invention, and FIG. 18 is an explanatorydiagram that shows which nozzle records each pixel of each raster linefor the sixth embodiment. The difference between this and theaforementioned first through fifth embodiments is that the value ofintermittence level q is increased from 2 to 4, and that the number ofworking nozzles is also increased. As shown in FIG. 18, each raster lineis recorded by 8 or 7 nozzles. With this sixth embodiment, by having theintermittence level q increased to 4, even if the main scan speed isincreased to twice that of the aforementioned first through fifthembodiments, the nozzle drive frequency does not increase. Therefore,from the perspective of the upper limit of the nozzle drive frequency,it is possible to increase the main scan speed to twice that of theaforementioned first through fifth embodiments. The increase in thenumber of woking nozzles also links to an increase in the number ofworking nozzles for recording one raster, making a further decrease inbanding possible.

As can be seen from the above embodiments, for the present invention,generally, it is preferable to use an original drive signal waveformthat allows formation of dots at a rate of one dot position in q on eachraster line where q is a designated integer of 2 or greater. This isbecause by increasing the main scan speed, it is possible to compensatethe decrease in printing speed that comes with an increase in the numberof overlaps. At this time, the raster data allocated to each nozzle(FIGS. 11(A) and 11(B)) is configured so as to allow each nozzle tointermittently form dots at a rate of one dot position in q or at a rateof one dot position in m×q where m is an integer of 2 or greater on eachraster line.

It is possible to think the aforementioned first through sixthembodiments as follows, from the point of the number of nozzles incharge of raster line recording. Specifically, with each of theaforementioned embodiments, the number of nozzles in charge of dotformation on some raster lines is set to a different value from thenumber of nozzles in charge of dot formation on the other raster lines.By doing this, it is possible to make fine adjustments in the balancebetween printing speed and banding reduction.

It is also possible to think the aforementioned first through sixthembodiments as follows, from the point of the recording target pixelratio of each nozzle. Specifically, with each of the aforementionedembodiments, i-th nozzle in the working nozzles can form dots atselected pixel positions on one raster line during one pass, but isactually permitted to form dots at a rate of one in m(i) selected pixelpositions. Further, the value of integer m(i) for at least two nozzlesis different form those for the other nozzles. For example, in the sixthembodiment shown in FIGS. 17 and 18, m(i) is 2 for nozzles :#1˜#10,#13˜#22, and 1 for nozzles #11, #12. By doing this, it is possible tomake fine adjustments in the balance between printing speed and bandingreduction.

Note that the above condition for the “number of nozzles in charge ofraster line recording” and the condition for the “recording target pixelratio of each nozzle” are not necessarily satisfied at the same time,and there are cases when only one is satisfied. For example, therecording target pixel ratio of each nozzle may not be fixed to aconstant value for each nozzle, and may change for each main scan. Evenin the case, the aforementioned condition for the “number of nozzles incharge of raster line recording” can be satisfied. The present inventionis applicable to these various cases.

F. Embodiment of Dot Recording Method That Removes Low Frequency ColorIrregularity

FIG. 19 is an explanatory diagram that shows a seventh embodiment of thepresent invention. This recording method is a non-uniform overlappingmethod with a constant feed as is the case with the first through thirdembodiments. However, the number of nozzles N and sub-scan feed amount Lare bigger than the first through third embodiments.

When sub-scan feed amount L gets greater, a nozzle pattern cycle getslonger. Here, a “nozzle pattern” means the array of nozzle numbers thatrecord one raster line. As can be seen from FIG. 19, with the constantfeed, the nozzle pattern is repeated at the cycle of sub-scan feedamount L. For example, the nozzle pattern for raster line #1 is#27-#14-#1-#14. For the #14 raster line, which is separated by thesub-scan feed amount L (13 dots) from raster line #1, also has the samepattern. Similarly, raster line #2 has the same nozzle pattern as rasterline #15, and raster line #3 has the same nozzle pattern as raster line#16. As explained below, if dots are deviated in the main scandirection, then image density variation and color shifts occur, andthere is a tendency for this image density variation to become morenoticeable as the sub-scan feed amount L becomes larger.

FIG. 20 is an explanatory diagram that shows a case when the dotmisalignment in the main scan direction. This kind of position shiftoccurs due to recording start position detection errors by the positionsensor 39 (FIG. 2) and nozzle manufacturing errors. Generally, withrasters recorded by a single nozzle, adjacent recorded dots are placedoverlapping correctly and regularly with each other in the rasterdirection. Meanwhile, with rasters recorded complementarily by multiplenozzles, gaps or overlaps occur between dots, causing image densityvariation. This image density irregularity also differs according to thenozzle pattern used for recording. Specifically, with the example shownin FIG. 20, the dots recorded on pass 3 are shifted to the right, so agap occurs between the dots on the third and sixth raster lines, and theimage density decreases.

FIG. 21 is a graph that shows the relationship between the spatialfrequency for human visual characteristics and the number ofdistinguishable tone. As shown in the Figure, as the spatial frequencygets larger recognition of the density difference becomes moredifficult. For example, with the first embodiment describe above,sub-scan feed amount L is 3 dots. Therefore, if we assume the rasterline density is 720 dpi, for example, then the spatial frequency ofsub-scan feed amount L is 9.4 cycles/mm (720 dpi÷(25.4 in×3 dots)). Inthis case, as shown in FIG. 21, because there are very few tones thatcan be distinguished, even if there is color irregularity for eachsub-scan feed amount L, it would be difficult for the human eye torecognize this.

However, as sub-scan feed amount L gets larger, the number ofdistinguishable tones rapidly increases, and color irregularity becomesnoticeable. For example, with the seventh embodiment, the spatialfrequency of the color irregularity that occurs for each sub-scan feedamount L is 2.2 cycles/mm (720 dpi÷(25.4 in×13 dots)). Therefore, in theseventh embodiment the color irregularity is more noticeable than in thefirst embodiment.

FIG. 22 is an explanatory diagram that shows the dot recording method ofan eighth embodiment of the present invention. This eighth embodimentdiffers from the seventh embodiment in that the nozzle count N of onecolor satisfies the following equations.

L=f×k±g  (1)

N=L+Rd[R×L÷k]  (2)

Here, L is sub-scan feed amount, f is an integer of 2 or greater, g isan integer of 1 or greater and less than k, and R is an integer that isgreater than k and not a integral multiple of k. In addition, Rd [ ] isan operator that is rounded down or rounded up. The reason why R is notan integral multiple of k is that when R is an integral multiple of k,the method is of uniform overlapping. With the eighth embodiment, k=3,f=4, L=13, and R=4. In addition, “+1” is selected as the value of theterm “±g” of equation 1.

The significance of equation (1) can be thought of as follows. When thesecond term “g” of equation (1) is ignored, the first term “f×k” makesthe sub-scan feed amount L. In this case, L consecutive raster lines aredivided into f sets of raster line groups each including k lines. Forexample, with the eighth embodiment shown in FIG. 22, 13 raster linesare divided into four raster line groups of 3 lines each. However, withthis eighth embodiment, the second term “±g” is “+1,” so the 13 rasterlines consists of four sets of raster line groups of 3 lines each andone raster line. Also, if the second term “±g” is set to 0, then L=f×k,so the conditions (for example the aforementioned condition c2) thatshould be satisfied by a constant feed recording method are notsatisfied. Specifically, the second term “±g” of equation (1) is forsatisfying a constant feed recording method. Also, when “±1” is used asthe second term of equation (1), there is the advantage that it ispossible to establish a constant feed recording method for any value aslong is k is 2 or greater.

The significance of equation (2) can be thought of as follows. The firstterm “L” at the right side of equation (2) shows the minimum nozzlecount for recording without overlapping. The second term Rd [R×L÷k] ofequation (2) shows the number of working nozzles for overlapping. TheL÷k here is a number that shows how many raster line groups are includedin the range of one sub-scan feed amount. Meanwhile, R is an integer, so“R×L+k” is a value that shows a integral multiple of the number ofraster line groups included in the range of one sub-scan feed amount. Ifthe rounding operator Rd [ ] of the second term is ignored, we can seethat the second term is intended to change nozzle count N by an integralmultiple of the number of raster line groups (L/k) included in the rangeof one sub-scan feed amount.

When R is 0, all raster lines are recorded without overlapping (in otherwords by one nozzle). Each raster line group includes k raster lines, sowith this non-overlapping recording, each raster line group is recordedby a total of k nozzles. Meanwhile, when R is 1 or greater, (R×L/k)nozzles are added for overlapping, but these (R×L/k) added nozzles canbe thought of as being approximately evenly allocated to each (L/k)raster line group. When this is done, R each added nozzles are allocatedto each raster line group. Therefore, when R is 1 or greater, eachraster line group is recorded by a total of (k+R) nozzles. With theexample in FIG. 22, k=3 and R=4, so each raster line group is recordedby a total of 7 nozzles. As can the aforementioned equation (2) has theeffect of evening the nozzle count used for recording each raster linegroup.

In this eighth embodiment, the reason that low frequency colorirregularity is removed is as follows. As can be seen from theexplanation above, each raster line group is equally recorded by 7nozzles. Also, the number of working nozzles to record each raster linein each raster line group is fixed at 3, 2, and 2. Also, hereafter, thenumber of working nozzles to record each raster line is called the“raster line recording nozzle count.” It is known that colorirregularity is also dependent on the raster line recording nozzlecount. With the eighth embodiment, the raster line recording nozzlecount in the raster line group is set at 3, 2, and 2, so we can think ofcolor irregularity as also occurring at a small cycle of k linescorresponding to the raster line group. As a result, the long cyclecolor irregularity that is easily visually recognized by humans iseliminated.

Meanwhile, with the seventh embodiment shown in FIG. 19, the parametersare the same as the eighth embodiment except for the nozzle count, butthere is the difference that nozzle count N is two less. As a result, asis clear from FIG. 19, the total nozzle count used by raster line groupsis 7, 7, 6, and 6, and this is not fixed. As a result, there is a chancethat color irregularity will occur at the long cycle of sub-scan feedamount L.

FIG. 23 is an explanatory diagram that shows the dot recording method ofa ninth embodiment of the present invention. With this ninth embodiment,other than the fact that R is 5, the parameters are the same as theeight embodiments. For this ninth embodiment as well, the number ofworking nozzles for recording each raster line group is fixed, and lowfrequency color irregularity is eliminated.

FIG. 24 is an explanatory diagram that shows the dot recording method ofa tenth embodiment of the present invention. This tenth embodiment isthe same as the eight embodiment described above except for the factthat “−1” is used as the value of the second term “±g” of equation (1).However, with this embodiment as well, the raster recording nozzle countin the raster line group is fixed at 3, 2, and 2, so low frequency colorirregularity is eliminated.

Also, with this tenth embodiment, −1 is selected as the value of theterm “±g” of equation (1), so one raster line is missing from the rasterline groups, and the fourth group in the range of sub-scan feed amount Lis not complete. In other words, the tenth and eleventh raster lines arethe first and second raster lines of the fourth group that were supposedto form, and its third raster line is missing.

Rounding up or rounding down can be used for rounding operator Rd [ ] ofthe second term of equation (2). Nozzle count N must be an integer, sothe rounding operator is used to make the calculation result an integer.This rounding up or rounding down generally affects the raster overlapcount on the raster lines outside the complete raster line groups, suchas surplus raster lines and raster lines in the partially missing rasterline group. For example, for the eighth embodiment, rounding down isused for Rd and the nozzle count N is 30, so the #13 raster line isrecorded by two nozzles. However, if rounding up is used for Rd and thenozzle count N is 31, then the #13 raster line is recorded by threenozzles. In addition, for example, for the tenth embodiment, rounding upis used for Rd and the nozzle count is 26, so the #10 raster line isrecorded by three nozzles. However, when rounding down is used for Rdand the nozzle count N is 25, raster line #10 is recorded by twonozzles.

This rounding up or rounding down may also affect the number of rasteroverlap count outside this surplus rasters and partially missing rasterline groups. For example, rounding up is used with the ninth embodiment(FIG. 23) described above, but if rounding down is used, rather than the#13 raster line that is the surplus raster, the overlap count of theadjacent #12 raster line will decrease. However, in this case, thenumber of working nozzles for the raster line group will decrease, butthe overlap count of the adjacent #13 raster line is 3, so the existenceof this raster line must be considered. When we consider replacing this#13 raster line with the #12 raster line, which is in a position soclose that it is almost unrecognizable by the human eye, the nozzlecount used for recording this raster line group is essentially equal tothe other raster line groups. As a result, in this kind of case as well,low frequency color irregularity is eliminated.

As explained above, with constant feed established by equation (1) whilelimiting nozzle count N by equation (2), it is possible to make thetotal nozzle count used for each raster line group essentially equal,and thus low frequency color irregularity is eliminated, making itpossible to improve image quality.

This kind of feature exhibits significant effects especially withbi-directional printing for which main scanning is performed in bothdirections. In other words, as described above, for bi-directionalprinting, with the outgoing pass, each color dot is formed on eachraster line in the sequence K_(D), C_(D), C_(L), M_(D), M_(L), andY_(D). Meanwhile, conversely with the return pass, each color dots areformed on each raster line in the sequence Y_(D), M_(L), M_(D), C_(L),C_(D), and K_(D). Because of this, with the raster lines recorded by theoutgoing pass and the raster lines recorded by the return pass, it ispossible to see a difference in color. However, the degradation of imagecaused by this color difference in the low frequency is also suppressedby this kind of feature.

FIGS. 25 through 27 are explanatory diagrams that show the dot recordingmethod of an eleventh embodiment of the present invention. Thedifference from the eighth through tenth embodiments is the fact thatthe sub-scan feed is variable feed. With variable feed as well, whensub-scan feed amount L gets larger, low frequency color irregularity canoccur the same as with constant feed. This color irregularity can alsobe removed by setting the nozzle count N for one color so as to satisfyequations (3) and (4) below.

L=Lave±g  (3)

N=Lave+Rd[R×Lave÷k]  (4)

Here, g is a positive integer that is 1 or greater and k or less, Laveis the average value of sub-scan feed amount L of one cycle, and R is aninteger that is greater than k but is not an integral multiple of k.

With this eleventh embodiment, sub-scan feed is done by variable feed of19-15-15-15 dot cycles. Therefore, average sub-scan feed amount Lave is(19+15+15+15)÷4, which is 15. Meanwhile, nozzle pitch k is 4. R isselected as any integer that is 5 or greater but not an integralmultiple of 4, and in this case, 5 is selected. As a result, nozzlecount N is set to 36.

With this eleventh embodiment, raster line groups constructed from kraster lines are formed, but the total nozzle count used to record foreach raster line group are all equal to 9. This eliminates low frequencycolor irregularity. However, with constant feed, the nozzle pattern isrepeated in a range of one sub-scan feed amount L, but with variablefeed, it is repeated in a range of one cycle of scans. Following, wewill explain the nozzle pattern for one cycle of sub-scan feed.

FIG. 25 shows pass 1 through pass 10 of the eleventh embodiment. Here,we will focus on the raster lines in the range from the raster linerecorded by the #1 nozzle on pass 9 to the raster line recorded by the#1 nozzle on pass 10, in other words, the #1 to #15 raster lines. Rasterlines #1 through #15, which corresponds to the range of the sub-scanfeed of 15 dots before pass 10, are divided into three raster linegroups recorded by an equal number of 9 nozzles and three residualraster lines that were originally supposed to be fourth raster linegroup. The reason that one raster line is missing from the fourth rasterline group is that the sub-scan feed amount before pass 10 is 1 dot lessthan 16 dots which is the integral multiple of k that is closest to theaverage sub-scan feed amount Lave.

FIG. 26 shows pass 2 through pass 12 of the eleventh embodiment. As canbe seen from this Figure and FIG. 25, the sub-scan feed amount beforepass 11 is 15 dots, so the raster lines #16 through #30, whichcorresponds to the range of the sub-scan feed of 15 dots are alsorecorded in the same way as the raster lines from #1 through #15. Also,the raster lines from #31 through #45 are also recorded in the same wayas the raster lines #1 through #15.

FIG. 27 shows pass 3 through pass 13 of the eleventh embodiment. Theseraster lines from #46 through #64 are formed by pass 4 through pass 12.The sub-scan feed amount before pass 13 is 19 dots, so the raster lines#46-#64 corresponding to the sub-scan feed are divided into four rasterline groups recorded by an equal number of 9 nozzles and residual threeraster lines. The reason that there are these three residual rasters isthat the sub-scan feed before pass 13 is 19 dots which is 3 dots higherthan 16 dots which is the integral multiple of k that is the closest tothe average sub-scan feed amount Lave.

FIGS. 28 through 30 are explanatory Figures that show the dot recordingmethod of a twelfth embodiment of the present invention. The differencefrom the eleventh embodiment is that the sub-scan feed is a 15-18-17-18dot variable feed. With this twelfth embodiment as well, nozzle count Nis set so as to satisfy equations (3) and (4). Therefore, as with theeleventh embodiment, the number of working nozzles for recording eachraster line group is fixed to 9 nozzles. This eliminates low frequencycolor irregularity.

As explained above, even when the sub-scan feed is variable feed, bysetting nozzle count N using equations (2) and (3), it is possible tomake the nozzle pattern of each raster line group the same, so lowfrequency color irregularity is eliminated, and the image quality can beimproved. Note that equation (4) is a more general form of equation (2).

G. Embodiment of Dot Recording Method with Variable Number of MainScanning

FIG. 33 is an explanatory diagram that shows the dot recording method ofthe thirteenth embodiment. This embodiment is same as the firstcomparative example of uniform overlapping method shown in FIG. 9 exceptthat additional passes are added to the first comparative example.

The additional passes and their nozzles are indicated with thesubscripts “a” attached to pass numbers. The additional pass is a passperformed at the same location in the sub-scan direction as itspreceding pass. The ink dots formed by the additional passes are alsoindicated with the subscripts “a” attached to the numbers in circles atthe right side of FIG. 33.

The parameters of this recording method are the same as those of thefirst comparative example except for the sub-scan feed. The sub-scanfeed of the first comparative example is “constant feeds” in whichsub-scan feeds are performed at constant amount of three dots. However,the sub-scan feed of this embodiment is “variable feeds” in whichsub-scan feeds are performed at a combination of multiple differentamounts. The combination is 0-3-3 dot cycle.

The combination of multiple different amounts includes the sub-scan feedamount of “0”. After the sub-scan of “0” or “no sub-scan”, theadditional passes are performed. For example, since no sub-scan feed ismade before the pass 1 a, pass 1 a is an additional pass. The pass 1 ais one of the additional passes, which implement the non-uniformoverlapping method.

The non-uniform overlapping method is implemented as described below.The number of passes for odd number raster lines are increased due tothe additional passes as shown in the right side of FIG. 33. On theother hand, each scan line (raster lines) is formed with two workingnozzles as in the case of the first comparative example. The non-uniformoverlapping method is implemented without increasing working nozzlesforming each scan line in this embodiment.

Not only the increase of working nozzles but also the increase of passescan improve image quality. Therefore, this type of overlapping methodcan improve the image quality. The number of passes for only odd numberraster lines are increased. Therefore, this recording method isnon-uniform overlapping method. This non-uniform overlapping method canbalance the image quality and the printing speed more finely than anuniform overlapping method. This balancing technique implements theimprovement of the image quality without excessively decreasing printingspeed.

FIG. 34 and FIG. 35 are explanatory diagrams that show the dot recordingmethod of the fourteenth embodiment. This embodiment differs from thethirteenth embodiment shown in FIG. 32 in that the additional passes areadded to the first embodiment of non-uniform overlapping method shown inFIG. 10.

As shown in FIG. 34 and FIG. 35, it is possible to combine “addition ofpasses” and “addition of nozzles” in order to implement the non-uniformoverlapping method shown in FIG. 10. This combination of overlappingtechnique provides further options to control the balance of the imagequality and the printing speed.

H. Variation Example

The present invention can be used not only for color printing but alsofor black and white printing. The present invention is also applicableto printing where each pixel is reproduced with a plurality of dots ofdifferent sizes. The present invention is farther applicable to drumtype printers. With a drum type printer, the drum rotation direction isthe main scanning direction, and the carriage scan direction is thesub-scanning direction. Also, the present invention can be used not onlyfor inkjet printers, but in general for dot recording apparatuses thatrecord on the surface of a printing medium using a recording head thathas multiple nozzle rows.

For the aforementioned embodiments, it is acceptable to replace part ofthe structure that is realized using hardware with software, andconversely, to replace part of the structure that is realized usingsoftware with hardware. For example, part or all of the functions ofprinter driver 96 shown in FIG. 1 can be executed by control circuit 40within printer 20. In this case, part or all of the function of computer90 that is the printing control apparatus that creates print data isrealized by control circuit 40 of printer 20.

When realizing part or all of the functions of the present inventionusing software, that software (computer program) can be provided in aform stored on a computer-readable storage medium. For the presentinvention, “a computer-readable storage” is not limited to a portabletype recording medium such as a floppy disk or CD-ROM, but also includesinternal memory devices in the computer such as various types of RAM andROM, or external memory devices connected to a computer such as a harddisk.

Although the present invention has been described and illustrated indetail, it is clearly understood that the same is by way of illustrationand example only and is not to be taken by way of limitation, the spiritand scope of the present invention being limited only by the terms ofthe append claims.

What is claimed is:
 1. A method of printing by forming ink dots on aprint medium, comprising the steps of: (a) providing a print head havinga plurality of nozzles arrayed along a sub-scanning direction forejecting same ink; (b) allocating n(j) number of nozzles to a j-th mainscan line in a selected area on the print medium where n(j) is aninteger of at least 2; (c) positioning one of the n(j) number of nozzleson the j-th main scan line during each of k(j) number of main scans tobe performed on the j-th main scan line where k(j) is an integer of atleast 2, the integer k(j) for some main scan lines being set at adifferent value from that for other main scan lines; and (d) driving theone of the n(j) number of nozzles for each of the k(j) number of mainscans, in response to given print data, to enable the nozzle to formdots intermittently at a rate of one in m×q dot positions on the j-thmain scan line during one main scan, m being an integer of at least 1,and q being an integer of at least 2, to thereby complete dot formationon the j-th main scan line during k(j) number of main scans.
 2. A methodin accordance with claim 1, wherein the k(j) is an integer of at leastn(j)+1.
 3. A method in accordance with claim 2, wherein the integer n(j)for some main scan lines being set at a different value from that forother main scan lines.
 4. A printing apparatus for forming ink dots on aprint medium, comprising: a print head having a plurality of nozzlesarrayed along a sub-scanning direction for ejecting same ink; a mainscan drive mechanism configured to relatively move a selected one of theprint head and the print medium to effect main scanning; a sub-scandrive mechanism configured to relatively move a selected one of theprint head and the print medium to effect sub-scanning; a head driverconfigured to drive the print head to eject ink; and a controllerconfigured to control the main scan drive mechanism, the sub-scan drivemechanism, and the head driver in response to given print data such thatdot formation on a j-th main scan line in a selected area on the printmedium is executed by n(j) number of nozzles during k(j) number of mainscans where n(j) is an integer of at least 2 and k(j) is an integer ofat least n(j), each of the n(j) number of nozzles being allowed to formdots intermittently at a rate of one in m×q dot positions on the j-thmain scan line during one main scan, where m is an integer of at least 1and q is also an integer of at least 2, to thereby complete dotformation on the j-th main scan line with the n(j) number of nozzlesduring k(j) number of main scans, the integer k(j) for some main scanlines being set at a different value from that for other main scanlines.
 5. A printing apparatus in accordance with claim 4, wherein thek(j) is an integer of at least n(j)+1.
 6. A printing apparatus inaccordance with claim 5, wherein the integer n(j) for some main scanlines being set at a different value from that for other main scanlines.
 7. A printing control apparatus for generating print data to besupplied to a printing unit to perform printing, the printing unitcomprising a print head having a plurality of nozzles arrayed along asub-scanning direction for ejecting same ink, the printing controlapparatus comprising: a print data generator configured to generate theprint data to effect printing such that dot formation on a j-th mainscan line in a selected area on the print medium is executed by n(j)number of nozzles during k(j) number of main scans where n(j) is aninteger of at least 2 and k(j) is an integer of at least n(j), each ofthe n(j) number of nozzles being allowed to form dots intermittently ata rate of one in m×q dot positions on the j-th main scan line during onemain scan, where m is an integer of at least 1, and q is an integer ofat least 2, to thereby complete dot formation on the j-th main scan linewith the k(j) number of main scans, the integer k(j) for some main scanlines being set at a different value from that for other main scanlines.
 8. A printing control apparatus in accordance with claim 7,wherein the k(j) is an integer of at least n(j)+1.
 9. A printing controlapparatus in accordance with claim 8, wherein the integer n(j) for somemain scan lines being set at a different value from that for other mainscan lines.
 10. A computer program product for causing a computer togenerate print data to be supplied to a printing unit to performprinting, the printing unit comprising a print head having a pluralityof nozzles arrayed along a sub-scanning direction for ejecting same ink,the computer program product comprising: a computer readable medium; anda computer program stored on the computer readable medium, the computerprogram comprising: a program for causing the computer to generate theprint data to effect printing such that dot formation on a j-th mainscan line in a selected area on the print medium is executed by n(j)number of nozzles during k(j) number of main scans where n(j) is aninteger of at least 2 and k(j) is an integer of at least n(j), each ofthe n(j) number of nozzles being allowed to form dots intermittently ata rate of one in m×q dot positions on the j-th main scan line during onemain scan, where m is an integer of at least 1, and q is an integer ofat least 2, to thereby complete dot formation on the j-th main scan linewith the k(j) number of main scans, the integer k(j) for some main scanlines being set at a different value from that for other main scanlines.
 11. A computer program product in accordance with claim 10,wherein the k(j) is an integer of at least n(j)+1.
 12. A computerprogram product in accordance with claim 11, wherein the integer n(j)for some main scan lines being set at a different value from that forother main scan lines.